Add RSR convert script for MERSI-1 on FY-3A/B/C

[yukaribbba]

Source:
https://img.nsmc.org.cn/PORTAL/NSMC/DATASERVICE/SRF/FY3A/FY3A_MERSI_SRF.rar
https://img.nsmc.org.cn/PORTAL/NSMC/DATASERVICE/SRF/FY3B/FY3B_MERSI_SRF.rar
https://img.nsmc.org.cn/PORTAL/NSMC/DATASERVICE/SRF/FY3C/FY3C_MERSI_SRF.rar

RSR:
rsr_mersi1_fy3abc.zip

Atmospheric correction:

[adybbroe]

LGTM. Many thanks for adding this support!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.00%. Comparing base (677af34) to head (d3b18d6).
Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #223   +/-   ##
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  Coverage   90.00%   90.00%           
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  Files          23       23           
  Lines        2571     2571           
=======================================
  Hits         2314     2314           
  Misses        257      257           

Flag	Coverage Δ
unittests	90.00% <ø> (ø)

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